Filtration of a coal liquid slurry using polyisobutylene

ABSTRACT

The rate of filtration of a coal liquid slurry is increased by adding polyisobutylene to the slurry prior to filtration or by washing a precoat cake of filter aid with an oil solution of polyisobutylene prior to filtration.

This invention relates to a process for the filtration of a coal liquidslurry.

Several solvation processes are now being developed for producing bothliquid and solid demineralized hydrocarbonaceous fuels from coal. Onesuch process is known as the Solvent Refined Coal (SRC) process. The SRCprocess is a solvation process for producing dissolved liquid and solidhydrocarbonaceous fuels from coal. In this process, crushed raw coal isslurried with a solvent comprising a recycle liquid fraction containinghydroaromatic compounds and is passed together with hydrogen through apreheating zone at an elevated temperature and pressure to dissolvehydrocarbonaceous fuel from coal minerals and to preventrepolymerization of dissolved hydrocarbons by transfer of hydrogen fromthe hydroaromatic solvent compounds to the dissolved hydrocarbonaceousmaterial. The resulting slurry is then passed to a second and exothermicdissolver zone wherein hydrogenation and hydrocracking reactions occur.Hydroaromatic compounds are formed in the dissolver zone and a fractioncontaining these compounds is recovered from the dissolver zone effluentslurry for recyle. The residue of the dissolver effluent slurry containsparticles of coal minerals and undissolved coal suspended in normallyliquid and normally solid dissolved coal, i.e., dissolved coal which issolid at room temperature. The suspended particles are very small, somebeing of submicron size with most or nearly all being smaller than 10 or20 microns in diameter. Because of their small size, these particles arevery difficult to filter or otherwise remove from the dissolved coal.

The present inventors have disclosed in U.S. Pat. Nos. 4,102,774 and4,124,485 the treatment of the effluent slurry from a coal liquefactionprocess, such as the SRC process, with an alcohol to agglomerate orotherwise affect the suspended or dispersed mineral-containing solids toincrease the filtration rate of the slurry.

We have now discovered that certain polymers, includingalkylmethacrylate or polyalkylmethacrylate copolymers, such asethylene-propylene methacrylate copolymer, ethylene vinyl acetatecopolymer and polyisobutylene are capable of increasing the filtrationrate of coal liquid slurries. The polyisobutylene is specific to thepresent application, while the alkylmethacrylate copolymer and ethylenevinyl acetate copolymer, respectively, are specific to applicationscofiled herewith. These polymeric materials are effective for increasingthe filtration rate when they are added directly to the coal liquidslurry and homogeneously dispersed therein at a concentration whichincreases the subsequent rate of filtration. Advantageously, the polymeris first dissolved in a light oil to reduce its viscosity. Furthermore,we have surprisingly found that these polymers are as or even moreeffective when they are dispersed or dissolved in a coal or petroleumoil in which they are soluble to reduce their viscosity and passedthrough a precoat cake of filter aid prior to the filtration of a coalliquid slurry therethrough. The achievement of an improvement infiltration rate by means of precoat treatment is particularly remarkablebecause we have found that the polymer is not irreversibly adsorbed onthe filter aid as a result of the pretreatment. The advantage of theinvention thereby can be achieved with an essentially complete recoveryof the polymer, or a recovery of most or at least 70, 80 or 90 weightpercent thereof. The polymer economy advantage prevails only in the caseof the precoat treatment mode of this invention, because the addition ofthe polymer to the coal slurry itself results in a loss of the polymerin the coal liquid filtrate. However, even this is preferably to loss ofthe polymer by adhesion to the filter aid, where it could not evencontribute to the combustion heating value of the coal filtrate.

The fact that these polymers impart a substantial improvement in thefiltration rate simply by the use thereof to wash a filter precoat cakewithout any significant amount of polymer remaining on the filter cakeindicates that these polymers affect the surface of the particles of theprecoat material in a physical rather than a chemical manner. Althoughthe polymer itself is relatively viscous, it is dissolved in a light oilto provide a pourable solution capable of flowing over the solidparticles of filter aid. Any oil remaining on the filter aid willsubsequently dissolve in the coal liquid during the filtration. Nosetting time is required between the polymer wash and the filtrationstep, but a time lapse is not harmful. The polymer wash exerts novisible effect upon the precoat cake and probably affects the interfacebetween deposited coal solids and the particles of filter aid during thesubsequent filtration step. It may be that the polymer pretreatmentprevents or reduces adhesion between the coal solids and the precoatsolids. Such an effect is highly distinctive from the apparent suspendedsolids agglomeration function of the alcohols employed in theaforementioned patents. Because of the apparent physical rather thanchemical nature of the effect, any solid commercial filter aid can beemployed in the practice of this invention. Examples of typical filteraid materials include diatomaceous earth, cellulose, asbestos, andslagwool.

We have further found that a synergistic effect upon filtration rate isachieved by employing with the polymer of this invention an alcohol ofthe aforementioned patents U.S. Pat. Nos. 4,102,774 and 4,124,485, whichare hereby incorporated by reference. To obtain the synergistic effect,the polymer can be added directly to the coal liquid slurry togetherwith the alcohol or a polymer in oil solution can be used to wash theprecoat cake so that only the alcohol is added directly to the coalliquid slurry. The discovery of the synergistic effect is a strongindication that the polymer and the alcohol perform unlike functions inthe filtration system.

The polymer of this invention can be added to a coal liquid slurry inany amount which provides an improvement in the filtration rate of theslurry. In general, an effective amount of polymer in the coal liquidslurry will be between 0.01 or 0.05 and 2 weight percent. Moreparticularly, the amount will be between 0.1 or 0.25 and 1 or 1.5 weightpercent. The polymer or this invention or an oil solution thereof can beadded to a coal liquid slurry even if an oil solution of the polymer isalso used to wash a precoat cake of filter aid.

The polymer can be added to the coal liquid slurry directly or added asa solution in an essentially solids-free coal or petroleum oil. When thepolymer is used to pretreat the filter cake, its viscosity must bereduced and therefore it is passed through the precoat cake in solutionwith a hydrocarbon oil. Solutions of polymer in a coal or petroleumhydrocarbon oil will generally contain between about 0.1 and 7 weightpercent of polymer and more particularly will contain between about 0.5and 2.5 weight percent of polymer.

Primary, secondary or tertiary aliphatic alcohols containing 2 to 10carbon atoms can be employed to obtain the aforementioned synergisticeffect. Although longer aliphatic chains may be effective, they are moreexpensive and needlessly increase the cost of the operation.Particularly effective alcohols include isopropyl and normal, secondaryand tertiary butanol. One or more alcohols can be employed. The alcoholcan be present in the coal liquid in an amount between 0.05 and 15weight percent. Alcohol concentration ranges between 0.1 and 15 weightpercent or between 0.5 or 1.0 and 6 weight percent are effective.

The alcohol employed does not perform any significant hydrogen donor orcoal solvation function. For example, while butanol is a preferredalcohol for purposes of filtration it is not an effective alcohol forpurposes of coal solvation. In the present process, the alcohol is addedto the coal liquefaction process after completion of the coal dissolvingstep, i.e., after at least about 85 or 90 weight percent of the coal hasbeen dissolved. There is no need to add alcohol to the process untilafter the coal dissolving and hydrogenation steps are completed.Furthermore, the alcohol in this process does not impart any significantincrease in the hydrogen to carbon ratio of the coal liquid. Thereby,most of the alcohol is not consumed in the present process, nor is theresignificant conversion to another material, such as a ketone, byhydrogen transfer. To prevent the alcohol from functioning as a hydrogendonor, the coal liquid to which the alcohol is added comprises asignificant amount of previously added and different hydrogen donormaterials, such as at least 2, 3 or 5 weight percent of hydroaromaticmaterials, such as tetralin and homologues thereof. The hydroaromaticmaterial conserves the alcohol so that most of it can be recycledwithout hydrotreatment. Since the purpose of the alcohol is specific tosolids removal, no prior removal of solids from the coal is required andthe alcohol can be added to a coal liquid slurry containing generally atleast 3 to 4 weight percent of minerals. The alcohol does not require abase in order to perform its function, such as would enhance its effectif it were to perform a hydrogen donor function. Also, the alcoholfunctions in the present invention in the liquid phase and therefore canbe used for solids-liquid separation at a temperature below its criticaltemperature.

The temperature of the coal liquid should be at an elevated level priorto alcohol addition and should be between about 100° and 700° F. (38°and 371° C.), generally, between about 150° and 600° F. (66° and 316°C.), preferably; and between about 400° and 500° F. (204° and 288° C.),most preferably. Following the addition of alcohol the coal mixtureshould be mixed to form a homogeneous composition within the liquidphase. After the addition of the alcohol and before the solids removalstep, the coal solution can be allowed to stand at the mixingtemperature from 30 seconds to 3 hours, generally, from 1 minute to 1hour, preferably, or from 2 or 5 minutes to 30 minutes.

An additional beneficial effect can be achieved when the alcoholadditive is in blend with a light oil. The light oil can be asubstantially solids-free light coal liquid fraction from which thesolids have been removed by filtering or other means, such as a processlight oil fraction whose boiling range includes the boiling temperatureof the alcohol. The blend can be recovered from the process as a singlefraction, or the light oil and alcohol can be removed separately fromthe process and then blended in any desired ratio. An alcohol-oil blendimparts a more beneficial effect upon filtration of solids from a coalliquid than an alcohol itself. While the advantage incident to theaddition of the alcohol by itself declines as the quantity of alcoholadded increases beyond a critical value, enhanced quantities of alcoholcan be employed with advantage by utilizing a blend of alcohol and lightoil. Since the alcohol is recycled, there is very little additionaloperating cost incident to the use of an enhanced quantity of alcohol.Phenol, which is present in coal liquids, has a detrimental effect uponsolids separation, apparently acting as a dispersion medium. In order toavoid recycle of phenol, the light oil fraction should boil below theboiling point of phenol, which is 358° F. (181° C.). For example, a coalliquid fraction boiling no higher than about 355° F. (169° C.) can beemployed. The boiling range of the coal liquid fraction need not overlapthe boiling range of recycle process solvent. This upper temperaturelimitation does not apply if the light oil is not a coal liquid, andtherefore does not contain phenols. For example, if the light oil is apetroleum fraction, a light, medium or heavy naphtha boiling no higherthan 500° F. (260° C.) can be employed. The amount of alcohol present inthe light oil fraction can be between about 1 and 75 weight percent,generally, or between about 10 and 25 weight percent preferably. Theamount of solids-free light oil-alcohol blend added to thesolids-containing coal liquid can be between about 1 and 50 weightpercent, generally, between about 1 and 15 weight percent, preferably,and between about 2 and 5 weight percent, most preferably.

In one mode, alcohol can be added to a hot, unfiltered slurry ofdissolved coal and the mixture is stirred and allowed to age. It is thenpassed through a filter containing a diatomaceous earth precoat whichhad previously been washed with a solution of polymer in light oil. Thealcohol-containing filtrate, which is essentially free of polymer, isthen fractioned to recover a low boiling fraction containing at least aportion of the alcohol. This fraction is then recycled and mixed withfilter feed slurry, together with any make-up alcohol that may berequired. This mode of operation provides the synergistic effect of thepolymer and alcohol upon the filtration rate, while allowing maximumeconomy in consumption of polymer and alcohol.

In performing the filtration tests to illustrate the present invention,the data obtained were interpreted according to the following well knownfiltration mathematical model:

    (T/W)=kW+C

where:

T=filtration time, minutes

W=weight of filtrate collected in time T, grams

k=filter cake resistance parameter, minutes/grams²

C=precoat resistance parameter, minutes/gram and

(T/W)=(rate)⁻¹

In the filtration tests reported below, the amount of filtraterecovered, W, was automatically recorded as a function of time, T. W andT represent the basic data obtained in the tests. The followingvariables were held constant at desired levels wherever necessary toobtain comparative measurements: temperature, pressure drop across thefilter, precoat nature and method of application, precoat thickness, andthe cross-sectional area of the filter.

The W versus T data obtained were manipulated according to the abovemathematical model, as illustrated in FIGS. 1 and 2. FIGS. 1 and 2 eachshow four curves, with each curve representing a separate series offiltration tests. The vertical axis of each figure shows the value forT/W, which is the reciprocal of the filtration rate. The slope of eachcurve is k, and the intercept of each curve with the vertical axis is C.

In analyzing each curve, the parameter C is primarily a characteristicof the precoat because it is the reciprocal of the filtering rate at thebeginning of the test before any significant amount of filter cake isdeposited on top of the precoat. On the other hand, the slope k is aparameter of the filter cake which is being deposited upon the precoatduring the filtration, and is therefore representative of the filtrationitself exclusive of the precoat. A relatively low slope (low value fork) represents an advantageously low cake resistance to filtration.Stated in another manner, any reduction in k represents an increase inthe prevailing rate of filtration. FIGS. 1 and 2 show that the linesrepresenting the base test and the test employing precoat treatment onlyhave the greatest slope (highest k) while the lines representing thetests utilizing an alcohol additive have the lowest slope (lowest k).Viewed in another manner, although each curve indicates a lowerfiltration rate (i.e., higher (rate)⁻¹) at the end as compared to thestart of a test, a low curve slope indicates that the filtering rate hasnot diminished greatly during the test. FIGS. 1 and 2 show that althoughthe lines representing the tests employing the precoat treatment onlyhave about the same slope as the line representing the correspondingbase test, the reduced precoat resistance parameter C in the case of thetests employing polymer treatment only resulted in a greater amount offiltrate after one minute as compared to the corresponding base test. Itis therefore apparent that the synergistic effect due to the employingof both precoat treatment and alcohol addition is due to thedifferentiated combined effects of reducing both the filter cakeresistance parameter k and the precoat resistance parameter C.

It is noted that each filtering test was performed without solventwashing of the filter cake. Since a solvent wash is intended to alterthe nature of the filter cake, it would also alter the k value. Manyindustrial filters are of the continuous rotary type wherein filtrationcycles of no more than about 1 minute duration are continuouslyalternated with washing cycles wherein a wash solvent is sprayed throughthe filter cake to wash off the absorbed coal liquid. Therefore, unlessotherwise indicated, all the tabulated filtering rates in the testsreported in the following examples represent the filtering operationduring the first minute of filtration.

In performing the filtration tests of the following examples, a 90 meshscreen located within the filter element was precoated to a depth of 0.5inch (1.27 cm) with diatomaceous earth. The filter element measured 1.9cm I.D. by 3.5 cm height and provided a surface area of 2.84 cm². Thescreen was supported by a sturdy grid to prevent deformation. Theprecoat operation was performed by pressuring a 5 weight percentsuspension of the precoat material in process light oil onto the screenusing a nitrogen pressure of 40 psi (2.8 kg/cm²). The precoat operationwas performed at a temperature close to that of the subsequent filteringoperation. The resulting porous bed of precoat material weighed about1.2 grams. After the precoat material had been deposited, nitrogen at apressure of about 5 psi (0.35 kg/cm²) was blown through the filter forabout 1-2 seconds to remove traces of light oil. The light oil flowed toa container disposed on an automatic weighing balance. The light oil wasweighed to insure deposition of the required quantity of precoatmaterial. Following this operation the light oil was discarded. Thebalance was linked to a recorder to be used later to provide acontinuous (at 5 second intervals) printed record of filtrate collectedas a function of time.

A 750 gram sample of unfiltered oil (UFO) without any additive was thenintroduced into a separate autoclave vessel which acted as a reservoir.The UFO was maintained at a temperature of 100°-130° F. (38°-54° C.) andwas continuously stirred. Stirring was accomplished using two 5 cmturbines. The shaft speed was 2,000 rpm. The filtration was begun byapplying a selected 40-80 psi (2.8-5.6 kg/cm²) nitrogen pressure to theautoclave. The UFO flowing from the autoclave passed through a preheatercoil whose residence time was controlled by the manipulation of valvesand which was provided with inlet and outlet termocouples so that theUFO reaching the filter was maintained at a uniform temperature. The UFOleaving the preheater was passed to the filter where a solid cake wasformed and a filtrate obtained. The filter element and filter heaterwere also fitted with thermocouples. As indicated above, filtrate wasrecovered on a balance and its weight was automatically recorded every 5seconds. The filtrate was collected in a clean container.

Comparative tests to determine the effect of additives were performedusing the same feed lot of UFO for which filtration data had beencollected. First, the system tubing and the filter were purged of UFOwith nitrogen at a pressure of about 100 psi (7 kg/cm²). The additivesubstance was pumped into the autoclave reservoir containing UFO. Aseparate filter element was fitted and precoated in the same manner asdescribed above and the tests employing an additive in the UFO wereperformed in the same manner as the tests performed on the UFO withoutan additive. Following each filtration, the residue on the precoatmaterial in the filter was purged with nitrogen and washed with anappropriate liquid to eliminate the UFO and additive combination.

Following is an analysis of a typical unfiltered SRC feed coal liquidslurry employed in the tests. Although light oil had been flashed fromthe coal liquid slurry in process pressure step-down stages and would beavailable for preparing polymer and alcohol blends, if required, thefilter feed oil had not experienced removal of any of its solids contentprior to filtration.

Specific gravity at 60° F. (15.6° C.); 1.15

Kinematic viscosity at 210° F. (98.9° C.); 24.1 centistokes

Density at 60° F. (15.6° C.); 1.092

Ash, 4.49 weight percent

Pyridine insolubles, 6.34 weight percent

Distillation, ASTM D1160

    ______________________________________                                        Percent         Temp. °F.                                                                        (°C.) at 1 atm.                              ______________________________________                                        5               518       (270)                                               10              545       (285)                                               20              566       (297)                                               30              602       (317)                                               40              645       (341)                                               50              695       (368)                                               60              768       (409)                                               70              909       (487)                                               71-recovery of all                                                            distillables occurs                                                           at 925° F. (496° C.)                                            ______________________________________                                    

For tests reported below employing a light oil to prepare a blend of analcohol or polymer, following are typical specifications for the lightoil.

Specific gravity at 60° F. (15.6° C.); 0.830

Density at 60° F. (15.6° C.); 0.829

Kinematic viscosity at 100° F. (37.8° C.); 0.861 centistokes

Distillation, ASTM D-86 at 763 mm Hg

    ______________________________________                                        Percent         Temperature °F. (°C.)                           ______________________________________                                         5              162 ( 72)                                                     95              442 (228)                                                     EP              492 (256)                                                     ______________________________________                                    

EXAMPLE 1

Tests were performed to illustrate the effect upon filtration rateachieved by adding certain polymers to a coal liquid slurry and thenfiltering the slurry at a temperature of 127° C. with a pressure drop of5.6 kg/cm² in a filter having a precoat cake of diatomaceous earthfilter aid, similar to the filter in the test system described above.The polymers tested includes ethylene-propylene methacrylate copolymer,ethylene vinyl acetate copolymer and a low and a high viscositypolyisobutylene. Each polymer tested was dissolved in a light oil toform a light oil solution which was miscible with and capable ofhomogeneous dispersion throughout the coal liquid slurry. The results ofthese tests are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Effect of Polymer Additives on Filtration of Coal Slurry                                            k     C     Rate.sup.1                                  Slurry Additive and Concentration.sup.2                                                        PPM  (min/g.sup.2)                                                                       (min/g)                                                                             (g/min)                                     __________________________________________________________________________    Base run (polymers absent)                                                                     0    .0271 .23   3.2                                         Ethylene-propylene copolymer of                                               methacrylate     2,400                                                                              .0232 .16   3.8                                         Ethylene-propylene copolymer of                                               methacrylate     10,000                                                                             .0139 .08   6.2                                         Ethylene vinyl acetate copolymer                                                               2,500                                                                              .0220 .10   4.8                                         Ethylene vinyl acetate copolymer                                                               10,000                                                                             .0142 .13   5.0                                         Polyisobutylene.sup.3                                                                          2,500                                                                              .0155 .12   5.8                                         Polyisobutylene.sup.3                                                                          10,000                                                                             .0172 .23   3.5                                         Polyisobutylene.sup.4                                                                          2,500                                                                              .0203 .26   3.1                                         Polyisobutylene.sup.4                                                                          10,000                                                                             .0181 .12   5.4                                         __________________________________________________________________________     .sup.1 For the first minute of filtration.                                    .sup.2 Base run was made using coal liquid slurry containing 5% light oil     Each polymer was added as a solution in the same amount of light oil.         .sup.3 Viscosity at 100° C. of 2474 centistokes.                       .sup.4 Viscosity at 100° C. of 680 centistokes. ?                 

Each of the polymer additives illustrated in the tests of Table 1induced an improvement in the filtration rate of the coal liquid, ascompared to the base run. In the case of the ethylene-propylenecopolymer of methacrylate, the ethylene vinyl acetate copolymer and thepolyisobutylene having a viscosity of 680 centistokes at 100° C., thefiltration rate improved with an increase in the amount of polymerpresent. However, in the case of the isobutylene having a viscosity of2474 centistokes at 100° C., the smaller amount of polymer imparted thegreater improvement in filtration rate. Based upon these data, apreferred isobutylene polymer has a molecular weight such that thepolymer exhibits a viscosity at 210° F. (99° C.) of between about 604and 690 centistokes.

EXAMPLE 2

Tests were performed to determine whether the polymers which improvedfiltration rate are retained by either the filter cake deposited duringfiltration of a polymer-free coal liquid slurry or by a precoat cake ofcommercial diatomaceous earth precoat material when washed with asolids-free light oil solution of a polymer prior to any contact of acoal liquid slurry therewith. In these tests, an individual samplecomprising a polymer dissolved in a light oil was passed through apolymer-free cake of coal minerals and also through a polymer-free cakeof diatomaceous earth. The results of these tests are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Polymer Concentration                                                         in Light Oil, Weight Percent                                                                    Following                                                                     Contact With                                                                              Following                                                         Cake of Coal                                                                              Contact With                                               Initial                                                                              Minerals    Precoat Cake                                    ______________________________________                                        Polyisobutylene                                                               in light oil 0.70     0.70        0.64                                        Ethylene-propylene                                                            copolymer of                                                                  methacrylate in                                                               light oil    0.70     0.68        0.70                                        ______________________________________                                    

Table 2 shows that within expected limits of experimental error nosignificant quantity of either polyisobutylene or of ethylene-propylenecopolymer of methacrylate was retained by either a polymer-free filtercake of coal minerals or by a polymer-free precoat cake of diatomaceousearth when either is washed with an oil solution of the polymer.Therefore, the advantage in filtration rate demonstrated in Example 3,below, obtained by washing a filter precoat cake of diatomaceous earthwith an oil solution of polymer is obtained by wetting the cake with thepolymer without any significant retention of the polymer on the cake.Apparently, such wetting produces an effect at the interface between theprecoat solids and the coal solids when these two materials contact eachother during the subsequent filtration step. One theory is that polymerpretreatment of the precoat prevents the coal solids from adhering tothe precoat solids. Such an effect is in sharp contrast to particleagglomeration, which is the probable mechanism when an additive such asan alcohol improves filtration rate.

The tests of Table 2 indicate that the advantage of the invention can beachieved with an essentially complete recovery of polymer. Thisadvantage prevails only in the case of the precoat wash mode of thisinvention, because the addition of the polymer to the coal slurryresults in a loss of the polymer in the coal liquid filtrate.

EXAMPLE 3

Tests were performed to illustrate the effect upon filtration rateobtained by washing a precoat cake of diatomaceous earth filter aid witheither a light oil solution of ethylene vinyl acetate copolymer or alight oil solution of polyisobutylene. The polyisobutylene used had aviscosity lower than that of the low viscosity polyisobutylene of thetests of Table 1. The filtration tests employing a light oil solution ofethylene vinyl acetate copolymer were performed at a temperature of 232°C. with a filter pressure drop of 5.6 kg/cm². The filtration testsperformed employing a light oil solution of the low viscositypolyisobutylene were performed at a temperature of 188° C. with apressure drop of 5.6 kg/cm². The precoat material, in the base test orafter washing with 25 grams of the indicated light oil solution ofpolymer, was subsequently used for the filtration of a coal liquidslurry. In some tests the coal liquid slurry contained no additive andin other tests the coal liquid slurry contained an additive comprising ablend of isopropanol in light oil in the indicated amount based on coalliquid slurry. The light oil used in preparing the isopropanol blend wasa light petroleum oil. The light oil used in preparing the polymersolutions was either a light petroleum oil or a light coal liquidfraction. The results of these tests are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Effect of Polymer                                                             Treatment of Precoat                                                          Upon Filtration Rate                                                                                             Filtration rate                            Coal liquid slurry                                                                      Precoat Cake             after first minute                                                                     Improvement over                  and additive, if any                                                                    Wash         k (min/g.sup.2)                                                                     C (min/g)                                                                           (g/min)  base test, percent                __________________________________________________________________________    Coal liquid                                                                   slurry only                                                                             None         .0247 .42   2.1      --                                Coal liquid                                                                             2% ethylene vinyl acetate                                           slurry only                                                                             copolymer in light coal oil                                                                .0293 .08   4.6      119                               Coal liquid                                                                   slurry containing                                                             5 percent light                                                               oil and 2% iso-                                                                         2% ethylene vinyl acetate                                           propanol  copolymer in light coal oil                                                                .0162 .05   7.5      257                               Coal liquid                                                                   slurry only                                                                             None         .0558 .30   2.2      --                                Coal liquid                                                                             7% low viscosity polyiso-                                           slurry only                                                                             butylene in light oil                                                                      .0438 .05   4.3       95                               Coal liquid                                                                   slurry containing                                                             5 percent light                                                               oil and 2% iso-                                                               propanol  None         .0359 .23   3.0       36                               Coal liquid                                                                   slurry containing                                                             5 percent light                                                               oil and 2% iso-                                                                         7% low viscosity polyiso-                                           propanol  butylene in light oil                                                                      .0252 .05   6.3      186                               __________________________________________________________________________

A comparison of the data of Table 3 with the data of Table 1 shows thatthe improvement in filtration rate achieved by the method of polymerprecoat wash is even greater than the improvement in filtration rateachieved by the addition of polymer to the coal slurry. This isparticularly remarkable because the method of precoat wash was shownabove to permit recovery of all or essentially all of the polymer usedfor the pretreatment, whereas polymer added to the coal slurry is notrecoverable but is ultimately consumed as fuel together with the coalliquid.

The data of Table 3 show that precoat prewashing with a light oilsolution of polyisobutylene increased the filtration rate 95 percent.The data of Table 3 further show that in the absence of precoatprewashing with a polymer or of addition of polymer to the coal liquidslurry, the addition of the isopropanol solution to the coal liquidinduced a 36 percent increase in filtration rate, using a sample of thesame coal liquid that was employed in the polyisobutylene test. Asummation of the 95 percent rate improvement due to polyisobutylene washof the precoat and of the 36 percent rate improvement due to independentaddition of isopropanol solution to the coal liquid slurry wouldindicate a combined effect constituting a 131 percent rate increase.However, when addition of the isopropanol solution to the coal liquidslurry was combined with prewash of the precoat with the polyisobutylenepolymer, a filtration rate increase of 186 percent was achieved,indicating a substantial synergistic effect.

The data of Table 3 show that prewashing of the precoat cake ofdiatomaceous earth with a light oil solution of ethylene vinyl acetatecopolymer without direct addition of copolymer to the coal liquidincreased the filtration rate 119 percent, compared to the base test.This increase is of a similar order of magnitude compared to the similartest employing polyisobutylene. However, when addition of theisopropanol solution to the coal liquid is combined with precoat washingusing an oil solution of ethylene vinyl acetate copolymer, an increaseof 257 percent was observed due to the combined effects, which increaseis even greater than the increase in the similar test employingpolyisobutylene. Therefore, a considerable synergistic effect is alsoachieved when a light oil solution of an ethylene vinyl acetatecopolymer is used for precoat washing, when the precoat washing iscoupled with the use of an isopropanol additive in the coal slurry.

EXAMPLE 4

Tests were performed to determine the effect of the concentration in thelight oil of the polymer used to prewash the precoat material. In thesetests, several different concentrations of polyisobutylene in light oilwere tested for washing a precoat cake of diatomaceous earth prior to acoal liquid slurry filtration operation performed at a temperature of204° C. and a pressure drop of 6.3 kg/cm². The filter cake resistanceparameter and the filtration rate for the first minute of filtration arereported in Table 4. In the tests, the indicated solution concentrationof polyisobutylene was used to wet the precoat material prior to thefiltration of a coal slurry which itself contained no filtrationimprover additives.

                  TABLE 4                                                         ______________________________________                                        Effect of Washing Precoat with                                                Light Oil Solutions of Polyisobutylene                                        Polyisobutylene Concen-                                                                        C          Filtration Rate                                   tration in Light Oil-Wt. %                                                                     (min/g)    (g/min)                                           ______________________________________                                        0                .33        2.5                                               2                .14        4.4                                               3.5              .10        5.0                                               7                .07        5.4                                               ______________________________________                                    

Table 4 shows that progressive increases in the concentration of thepolymer in the light oil resulted in progressive increases in filtrationrate. However, there is a sharp decline in the effect of increases inpolymer concentration at levels above about 2 or 3 weight percent. Table4 shows that the filtration rate is doubled when the polymerconcentration used for the precoat wash is 3.5 percent, but doublingthis polymer concentration to 7 percent provided only a relatively smalladditional improvement in filtration rate.

EXAMPLE 5

FIG. 1 graphically illustrates the effect upon filtration rate ofprecoat prewetting with a light oil solution of an ethylene-propylenecopolymer of methacrylate while FIG. 2 graphically illustrates theeffect upon filtration rate or precoat prewetting with a light oilsolution of polyisobutylene. In each figure the effect upon thesubsequent coal liquid filtration rate is shown both with and without anadditive to the coal liquid slurry comprising 2 weight percentisopropanol and 5 weight percent light oil, based on the coal liquidslurry. FIGS. 1 and 2 each show a base run with no additive in the coalliquid and with no precoat pretreatment. FIG. 1 shows that separateprecoat pretreatment tests were performed with the methacrylatecopolymer, one test wherein the copolymer was dissolved in a lightpetroleum oil boiling in the range 162° to 340° C. and another testwherein the copolymer was dissolved in a light coal oil boiling in therange 361° to 463° C. FIGS. 1 and 2 show the filtration rates prevailingafter periods of 0.5, 1, 2, 3 and 4 minutes. The tests of FIG. 1 wereperformed at a temperature of 230° C. with a pressure drop of 5.6kg/cm². The tests of FIG. 2 were performed at a temperature of 188° C.with a pressure drop of 6.3 kg/cm².

The first minute of filtration has a particular significance in rotarydrum filtration systems wherein an outer skin of the filter cake isscraped off upon each revolution of the drum by means of a knife edge,and the filter generally rotates for less than one minute before newlydeposited filter cake reaches the knife edge. In this manner, long-termresidue accumulation upon the filter is prevented and a fixed or staticfiltration system does not develop. By scraping the surface of a rotaryfilter with a knife edge at intervals of less than 1, 2 or 3 minutes, arelatively fresh filtration system is continuously maintained. A 0.001inch (0.0024 cm) cut per revolution is generally adequate. FIG. 2graphically illustrates the data of Table 3 wherein it was indicatedthat at a 1 minute filtration time the effect of addition of the lightoil solution of isopropanol additive to the coal liquid alone provided a36 percent increase in the filtration rate while the effect of thepolymer precoat was alone provided a 95 percent increase in filtrationrate. Whereas the arithmetical summation of these individual effects is131 percent, the empirical combined effect shown in Table 3 is 186percent, indicating a synergistic effect upon concomitant performance ofthese two filtration features. These data strongly indicate that each ofthese effects is functionally different, the effect of the alcoholprobably being to agglomerate coal mineral particles in the coal slurrywhile the polymer probably exerting an interface effect between thedeposited coal solids and the particles of filter aid.

FIG. 1 shows that after a one minute filtration time the wash of theprecoat with a light oil solution of ethylene-propylene methacrylatecopolymer imparted a 90 percent or a 119 percent improvement in thefiltration rate, depending upon which light oil solvent is employed forthe polymer. However, the combined effect of the precoat prewash and theaddition of an isopropanol additive to the coal liquid slurry was aremarkable 257 percent, again indicating synergism and providing strongevidence that different functions are exerted by the isopropanol in thecoal liquid slurry and the polymer precoat wetting step.

EXAMPLE 6

Screening tests were performed wherein commercial polymers other thanthose tested in the previous examples were added to coal liquid slurriesto determine their effect upon filtration rate. These filtrationscreening tests were performed at a temperature of 250° C. with a filterpressure drop of 15 psi (1.05 kg/cm²) in a filtration system employing adiatomaceous earth precoat and similar to the test system employed inthe previous examples. The filter rate after a 12 minute test period foreach of these additives is presented in Table 5. A lengthy filtrationinterval was employed because these tests were screening tests made forthe purpose of eliminating obviously ineffective additives.

                  TABLE 5                                                         ______________________________________                                                           Concentration                                                                 in Coal Liquid                                                                Slurry, Weight                                                                            Flow Rate,                                     Additive           Percent     grams/min.                                     ______________________________________                                        None               0           0.88                                           Acrylate copolymer 2           0.71                                           Commercial anionic water                                                      soluble polymer    2           0.88                                           Chlorinated low MW polyethylene                                                                  1           0.85                                           Polyvinyl acetate copolymer                                                                      1           0.47                                           Commercial cationic water                                                     soluble polymer    2           0.72                                           ______________________________________                                    

The data of Table 5 show that certain polymers are not capable ofenhancing the filtration rate even though they are derived from monomersonly slightly different from monomers used to prepare a filtration rateenhancing polymer. For example, while methacrylate copolymer was shownabove to improve the filtration rate of coal liquids, the acrylatecopolymer did not. Also, while ethylene vinyl acetate copolymer wasshown above to improve the filtration rate of coal liquids, neither thepolyvinyl acetate copolymer derived from a monomer other than ethylenenor chlorinated low MW polyethylene were capable of improving thefiltration rate of coal liquids. Finally, neither the commercial anionicwater soluble polymer nor the commercial cationic water soluble polymerwas capable of improving the filtration rate of the coal liquid, whichis an essentially water-free slurry. These water soluble polymers areconsrasted to the non-water soluble polymers tested above includingpolyisobutylene, ethylene-propylene copolymer of methacrylate andethylene vinyl acetate copolymer, each of which is capable ofhomogeneous dispersement in a coal hydrocarbonaceous liquid either aloneor in solution with a solubilizing oil, such as an oil derived from thecoal liquid.

EXAMPLE 7

Further screening tests were performed wherein additional commercialpolymers were added to coal liquid slurries to determine their effectupon filtration rate. These filtration tests were performed at atemperature of 550° C. with a filter pressure drop of 22 psi (1.54kg/cm²) in a filtration system employing a diatomaceous earth precoatand similar to the test system of the previous examples. Because thesetests were screening tests, the filter rate was measured after a 12minute test period. For comparative purposes two filtrationrate-promoter polymer additives, polyisobutylene and ethylene vinylacetate copolymer were tested over a similar period. The results ofthese tests are presented in Table 6.

                  TABLE 6                                                         ______________________________________                                                         Concentration                                                                 in Coal Liquid                                                                Slurry, Weight                                                                             Flow Rate                                       Additive         Percent      grams/min.                                      ______________________________________                                        None             0            1.88                                            Commercial cationic water                                                     soluble polymer  1            1.11                                            Resin composed of cyclo                                                       and alkyl saturates                                                                            2            1.90                                            Commercial cationic water                                                     soluble polymer mixture                                                       including polyamid resins                                                                      1            1.39                                            Polyisobutylene  1            2.90                                            Ethylene vinyl acetate                                                        copolymer        2            3.64                                            ______________________________________                                    

The data of Table 6 show that neither the commerical water solublepolymers nor the resin composed of cyclo and alkyl saturates exerted asignificant filter rate improvement effect in the essentially water-freecoal liquid slurry being filtered. In contrast, the non-water solublepolymers, polyisobutylene and ethylene vinyl acetate copolymer, eachexerted a substantial filter rate improving effect in these 12 minuteadditive screening tests.

We claim:
 1. A process for filtering a coal liquid slurry produced in aprocess for dissolving hydrocarbonaceous fuels from coal with a solventand containing hydrocarbonaceous liquid and suspended coal mineralscomprising adding polyisobutylene to said slurry in an amount whichincreases the rate of filtration thereof, and then filtering saidslurry.
 2. The process of claim 1 wherein a solution of polyisobutylenein hydrocarbonaceous oil is added to said slurry.
 3. The process ofclaim 1 wherein a solution of about 0.1 to 7 weight percentpolyisobutylene in hydrocarbonaceous oil is added to said slurry.
 4. Theprocess of claim 1 wherein the viscosity of said polyisobutylene isbetween about 604 and 690 centistokes at 210° F.
 5. The process of claim1 wherein the amount of polyisobutylene added to said slurry is betweenabout 0.01 and 2 weight percent.
 6. The process of claim 1 wherein theamount of polyisobutylene added to said slurry is between about 0.05 and1.5 weight percent.
 7. The process of claim 1 wherein saidpolyisobutylene is non-water soluble.
 8. A process for increasing therate of filtration of a coal liquid slurry produced in a process fordissolving hydrocarbonaceous fuels from coal with a solvent andcontaining hydrocarbonaceous liquid and suspended coal mineralscomprising depositing a precoat cake of filter aid on a filter element,passing a solution of polyisobutylene in hydrocarbonaceous oil throughsaid precoat cake, and then filtering said slurry through said precoatcake.
 9. The process of claim 8 wherein the solution of polyisobutylenein hydrocarbonaceous oil comprises between about 0.1 and 7 weightpercent of polyisobutylene.
 10. The process of claim 8 wherein theviscosity of said polyisobutylene is between about 604 and 690centistokes at 210° F.
 11. The process of claim 8 wherein said filteraid is diatomaceous earth.
 12. The process of claim 8 wherein saidpolyisobutylene is non-water soluble.
 13. The process of claim 8 whereinat least 70 percent of said polyisobutylene passes through said precoatcake.
 14. The process of claim 8 wherein essentially all of saidpolyisobutylene passes through said precoat cake.